1. Field of the Invention
The present invention relates to a display apparatus such as a color liquid crystal display apparatus that is capable of displaying a sharp color image by adjusting a luminance of the image while maintaining a hue of the image.
2. Description of the Related Art
A conventional color liquid crystal display apparatus generally includes three pixels for each dot. However, a recently proposed color liquid crystal display includes four pixels (RGBW) for each dot. In other words, a white (W) pixel has been added to the three pixels for each dot in the conventional color liquid crystal display for the purpose of improving luminance. This RGBW color liquid crystal display is advantageous in improvement of luminance. However, this RGBW color liquid crystal display tends to cause reduced color saturation. Accordingly, it is necessary to carefully consider how to determine an output tone and an arrangement of the W pixel.
For example, Japanese Unexamined Patent Publication No. 102292/2004 (Tokukai 2004-102292) (published on Apr. 2, 2004, and hereinafter referred to as Patent Document 1) discloses a W pixel arrangement. Patent Document 1 proposes to reduce a size of a W pixel so that the size becomes smaller than respective sizes of the RGB pixels. This proposal is intended to prevent the occurrence of reduced color saturation caused by a display signal of the W pixel.
Patent Document 1 also discloses that input RGB data is converted into RGBW data, and then the RGBW data thus converted is further converted into an optimum data R′, G′, B′, and W′. Although Patent Document 1 does not specifically explain how to convert RGB data into RGBW data, the further data conversion of the RGBW data into the R′ G′ B′ W′ data is intended to improve luminance and to maintain color saturation at the same time.
However, the more the size of the W pixel is reduced so that the size becomes smaller than the respective sizes of the RGB pixels, the more an effect of improvement in luminance is damaged. In an embodiment of Patent Document 1, a 50% increase in luminance is found but this is found only when all RGBW pixels have identical sizes. However, for example, in a case where R, G, and B are 1.05 and W is 0.85 under an assumption that an original size of each color pixel is 1, an increase in luminance is no more than approximately 42%. Moreover, a computing process in a drive circuit is further increased when carrying out two-step conversions, i.e., (1) a first step conversion of RGB input signals into RGBW signals, and (2) a second step conversion of the RGBW signals into an R′G′B′W′ signals. This causes an increase in a circuit size, and in the cost of the circuit.
Japanese Unexamined Patent Publication 241551/1993 (Tokukaihei 5-241551) (published on Sep. 21, 1993, and hereinafter referred to as Patent Document 2) discloses a simplest and easiest arrangement for calculating RGBW signals from RGB signals. This arrangement is characterized by including: means for extracting a white signal from a plurality of input color component signals (min detecting section 1); and means for outputting four color display signals including at least a white color display signal, in response to the extracted white color signal and the plurality of color component signals. Moreover, the method in Patent Document 2 is characterized by further including means for carrying out a nonlinear conversion with respect to the extracted white signal, and for outputting a display signal in response to (i) the plurality of color component signals and (ii) the white signal thus non-linearly converted.
In this method, the white signal W is extracted as a minimum value of the RGB signals, and the non-linear conversion of W into W′ is performed according to need. In the method, the following calculations are carried out.OUT(R,G,B)=IN(R,G,B)−OUT(W)OUT(W)=W or W′
According to this method, any one of RGB outputs inevitably becomes 0 if W is not subjected to the non-linear processing. Particularly when white or gray color is displayed, both of the equations Rout=Gout=Bout=0 and W=min (Rin, Gin, and Bin) are satisfied. Accordingly, with this method, luminance is not improved, and, moreover, the occurrence of reduced color saturation becomes significant. If the non-linear processing is carried out with respect to W so that W is converted into W′, a problem of the occurrence of reduced color saturation is solved to a certain extent. However, this does not lead to improvement of luminance, because luminance of an input signal is not extended.
A more complex method of calculating RGBW signals from RGB signals is disclosed, for example, in Japanese Unexamined Patent Publication 119714/2001 (Tokukai 2001-119714) (published on Apr. 27, 2001, and hereinafter referred to as Patent Document 3). Patent Document 3 proposes a method of obtaining a white signal component. The method includes the steps of (i) finding an increased luminance amount for each of fundamental color components each constituting a predetermined color signal, (ii) extracting an increased luminance amount of a white color signal component from the increased luminance amounts of the fundamental color components, and (iii) setting the increased luminance amount of the white signal component as a white signal component of the predetermined color signal.
A specific method that Patent Document 3 discloses is explained below with reference to FIG. 7. In FIG. 7, for simplification, only two signals of R and G are taken into consideration. For example, in a case where R is a minimum luminance signal Lmin and G is a maximum luminance signal Lmax, color A is extended to a maximum luminance A′ which is an extension of OA. Color C having the same R to G luminance ratio as the color A is also subjected to luminance extension performed with a luminance extending rate S=OA′/OA. Similarly, a luminance extending rate S=OB′/OB is applied to color D. A relationship between a luminance extending rate S and t (=Lmin/Lmax) becomes as shown in FIG. 8.
However, the conventional invention disclosed in Patent Document 3 has a problem that displayed image quality deteriorates. Namely, in the conventional invention, color saturation of a primitive color looks reduced when the primitive color is adjacent to white color. This is because luminance of primitive colors is not extended at all. This results in the problem of deterioration in image display quality. Moreover, as illustrated in FIG. 8, the luminance extending rate S is saturated at t=0.5 and a curve of the luminance extending rate S is turned. This causes a pattern where an image looks unnatural, thereby presenting the problem of deterioration in display image quality.